Is it possible to confirm if a frame is 4130 chromoly or 1020 steel ? I cannot find anything stamped into the frame other than "made in Japan".

Seatpost size can be an indication.

Steel seat tubes all have the same outside diameter. If the material is better quality (ie better than gas pipe), and strong enough to allow it, the wall thickness is reduced - so a bigger seatpost is required.

Gaspipe frames usually have 1" (25.4 mm) seatposts. If the post is bigger than that, you've got a thinner wall tube which is indicative of CrMo. 27.2 mm is a typical seatpost in a good CrMo frame.

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